Memory systems, optical or magnetic, have moving recording media on which information is recorded in sections or sectors. These information sections must be accessible so that desired information can be read therefrom. Accessibility is achieved by providing header sections of discrete recordings on the moving media ahead of these information sections, defining fields of bit code patterns, forming synchronizing fields, start of information or data fields, referred to as start of data marks, together with data address fields, which together contain information for providing timed access to the recorded information or data. These fields comprise recordings on the media in the form of bits, such as magnetic dibits on magnetic media, which are transduced as the media moves past a transducer disposed adjacent the surface of the media. The transducer is optically or magnetically coupled to the recorded bits on the media. The bits on magnetic media are serially recorded in patterns of transition and non-transition magnetic dibits. The magnetic media surface is usually of one state of magnetic polarization, which when read (transduced) within the clock pulse intervals (bit times), provides electrical indications which characterize the non-transition bits. Transition bits are oppositely magnetically poled from the non-transition bits in the magnetic media and therefore provide different electrical indications.
The header section in the form of bit patterns, may be used for various purposes. Among such purposes is the purpose to synchronize and phase lock the servo system and read write controls with respect to the media for reading and writing operations, and to provide an indication useful in identifying a particular location or position on a moving media at which information is to recorded or read by the transducer. The start of data mark is part of the header field. The header field may typically include in sequence, a write splice field, a phase lock field, a synchronizing field, a start of data mark or field and a data address field.
It is difficult in high density recording, to write codes comprising magnetic dibits, for example, without error, particularly a single bit error. Without defect tolerance in reading such codes, such as a start of data code, it is necessary to spare (skip over) any section or sector that contains a defect in the start of data mark. This is wasteful of data space. Also, if a dedicated servo is employed, drive timing is sufficiently inexact due to structural flexibility to cause reading errors. Thus timing shifts due to mechanical displacement between the servo head and the individual read-write or data heads, called head shift, and digital signal processor quantization errors, result in movement or displacement of the start of data mark whenever it is written. The end result is that a media surface area equal to the maximum range in which a start of data mark can be written, must be scanned and be totally defect free, in the absence of fault tolerance in the transducing system. Of course the longer the area, the higher is the probability of such a surface defect.
The longer the start of data mark, on the other hand, the easier it is to make the mark fault tolerant since there are more bit transitions to correlate. It is also desirable that the start of data mark be an integer number of bytes long (8 bits/byte), because the timing circuitry must already work in byte long increments so that no additional timing is required to access the start of data mark. A desirable goal is a start of data mark that is one byte long. In general, long start of data marks are undesirable because they increase the overhead in the header sections which reduces the user data storage capacity.
Prior art presently known to the applicant does not directly address the matters discussed above.
U.S. Pat. No. 4,740,941 entitled "System For Aligning Sector Marks With Data In A Disk Storage System", P. L. Shah et al, issued Apr. 26, 1988, describes a memory system using servo tracks and data tracks. Marks in the servo track define the sectors and marks in the data track define the beginning of data. The first sector mark in a track is an index mark. The beginning of data mark has a fixed location relative to the index mark. The read/write head is controlled to properly transduce the marks from which sector mark to data time differences for each track are determined. Fault tolerance in relation to bit defects is not addressed.
U.S. Pat. No. 4,584,616 entitled "Format For Storing Data On Magnetic Media", D. M. Allen, issued Apr. 22, 1986, describes a data track comprising in sequence, a header field, a data field and a track gap at the end of the data field. The header field contains encoded control information signalling the beginning of the data field. The track gap is a depository for spurious data from previously recorded data. The gap length is shorter than the header field to prevent its recognition as the header field. Fault tolerance in relation to bit defects is not addressed.
U.S. Pat. No. 4,297,737 entitled "Sector Servo With Sync Marks", R. Andresen et al, issued Oct. 27, 1981, employs an encoded sync signal to indicate the validity or invalidity of the servo information in that sector. Fault tolerance in relation to bit defects is not addressed.
U.S. Pat. No. 3,997,876, entitled "Apparatus and Method For Avoiding Defects In The Recording Medium Within A Peripheral Storage System", D. I. Frush, issued Dec. 14, 1976, addresses fault tolerance in relation to data by noting the location of surface defects in a disk in data recording areas and recording special codes thereat, which are recognized and disregarded as data during data reading. Fault tolerance in relation to bit defects is not addressed in this respect. The fault tolerance is directed to disk surface defects to prevent recording data on the surface defects.
Fault tolerance with respect to bit defects in the header field is needed to minimize skipping over and wasting of data space on a recording media.